Cultivation of animal cells has been increasingly important in both biochemical research and industrial production of hormones, vaccines, enzymes, and so forth. It is well known that in almost all of the methods of cultivating such cells it is essential for serum to be added to a culture medium. The serum, however, has disadvantages in that its supply is instable, uniform quality can be obtained only with difficulty, its storage and handling are difficult because it is liquid, and in that it is very expensive. This has seriously inhibited the development of cell cultivation in the biochemical research and the industrial production of, e.g., hormones.
In order to overcome the above-described problems, a number of investigations have been made to decrease the amount of serum necessary for cultivation by adding various kinds of hormones or proteins. Almost all of the investigations, however, are directed to a culture medium containing no serum (hereafter referred to as "serum-free culture medium") or a culture medium containing only a small amount of serum (hereinafter referred to as "low serum-content culture medium"), which are to be used in the cultivation of established cell lines and heteroploid cell lines.
Although these cells are often capable of growing limitlessly, many of them have a tumor-inducing ability and easily changeable properties. Difficulties, therefore, are encountered in their industrial application. On the other hand, normal diploid cells are cells originating from the normal tissues of animals, which have the number of chromosomes of 2n (e.g., 46 in the case of human being), undergo meiosis, and need a solid surface on which they are adsorbed. Since these normal diploid cells do not have a tumor-inducing ability and their properties are not easily changeable, they have been expected as stable cells which are suitable for use in commercial production of hormones, vaccines, enzymes, and so forth. The normal diploid cells, however, have complicated or specific nutritional requirements compared with established cell lines and heteroploid cell lines. A serum-free culture medium or a low serum-content culture medium, therefore, has not yet been established which is suitable for use in the cultivation of normal diploid cells.